c language interpretation, involves integer calculation

Question

I have the following code,

void main(void) {
    char x = 1 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 - 1;
    printf("%d", x);
}

I get the result -1

However, 1 * 2 * 2 * 2 * 2 * 2 * 2 * 2, already gets -128. Can you explain why I can successfully run the code above and get the result -1?

Also, is this an example of integer negation or overflow?

in many of implementation, char is by default signed type. — Sourav Ghosh
– Sourav Ghosh, Commented Apr 22, 2020 at 13:11
According to the C standard, void isn´t the right return type of main. It shall be an integer-compatible type or of course an int. — RobertS supports Monica Cellio
– RobertS supports Monica Cellio, Commented Apr 22, 2020 at 13:55

chux · Accepted Answer · 2020-04-22 15:11:37Z

6

1 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 is int math with a product of 256.
1 * 2 * 2 * 2 * 2 * 2 * 2 * 2 * 2 - 1 is 255.

So code is like

char x = 255;
printf("%d", x);

If a char is an unsigned char with the range of 0...255, the output is

If a char is a signed char with the range of -128...127, the assignment converts the 255 in an implementation defined manner to a char - likely a wrap around¹ (or 255 - 256) with a value of -1. The output is

-1

char is implemented as an unsigned char or signed char.
In OP's case, it is a signed char.
A signed char has the same range, size, encoding as signed char, yet is a distinct type.

is this an example of integer negation or overflow?

Neither. It is an example of an implementation defined conversion. And a very common one at that.

Conversions .... Otherwise, the new type is signed and the value cannot be represented in it; either the result is implementation-defined or an implementation-defined signal is raised. C17dr § 6.3.1.3 3

¹ Conceptually with 2's complement encoding, the least significant byte pattern of int 255 or 0000...0000_1111_1111₂ is preserved as signed char where the lead bit is a sign (or -128 place).

edited Apr 22, 2020 at 15:11

answered Apr 22, 2020 at 13:16

chux

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2 Comments

Gillian Over a year ago

Is integer warping the same as conversion?

chux Over a year ago

"integer warping " is an example of a conversion. In this case, from an int to a signed char. 123.4 saved as an char is 123 is truncation, another form of conversion, from double to char.

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